Method of manufacturing a refractory lined continuous casting die

ABSTRACT

A method of manufacturing a die for the continuous casting of metals comprising depositing a layer of refractory material on a disposable mandrel, depositing a support of thermally conductive material around and in intimate contact with the outer surface of the layer, and subsequently removing the mandrel.

D United States Patent [1 1 3,751,788 King 51 Aug. 14, 1973 METHOD OF MANUFACTURING A [56] References Cited REFRACTORY LINED CONTINUOUS UNITED STATES PATENTS CASTING DIE 2,173,484 9/1939 Lerch et al. 164/46 x 75 [nvemon Robert King, Sutton coldfield, 3,204,917 9/1965 Richards 164/24 X England 3,248,788 5/1966 Goldsteln et a] 29/423 [73] Assign, Imperial Mam Industries (Knock) FOREIGN PATENTS OR APPLICATIONS i i Birmingham, England 628,342 8/1949 Great Britain 164/24 [22] Filed frinszsfinsrfi: 5.1 9 5. M99 1. [211 App], 1%,; 173,55 Attorney-John W. Malley, Akin T. Davis et al.

I Related US. Application Data [57] s CT [62] 3:22;? 1968 A method of manufacturing a die for the continuous casting of metals comprising depositing a layer of refractory material on a disposable mandrel, depositing (5|. 29/423522 a pp of thermally conductive material around and [58] Fie'ld 4 34 36 in intimate contact with the outer surface of the layer,

and subsequently removing the mandrel.

=1 2 Drawing Figures METHOD OF MANUFACTURING A REFRACTORY LINED CONTINUOUS CASTING DIE This is a division of application Ser. No. 775,774 filed Nov. 14, 1968, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to the casting of metal and in particular to an apparatus and method of manufacture thereof for the continuous casting of strip, bar and plate of metals and metal alloys.

In the continuous casting of metals, molten metal is continuously introduced from a holding furnace or crucible into one end of a die and the metal is at least partly solidified within the die whilst passing therethrough before being continuously withdrawn from the other end of the die.

For continuous casting it is well known to employ a die made of graphite contained within a water-cooled metal jacket support. Graphite is particularly employed in the field of non-ferrous metal casting because it is inert to most non-ferrous metals and alloys and has the useful property of being self-lubricating. Moreover, graphite is easily machinable. However, it has limitations in use; die replacement is necessary after a limited service life so that truly continuous casting is not achieved.

It seems that the limitation of graphite arises from its porosity; even when very dense graphite is employed, it is nevertheless porous so that, when agraphite die is used in the continuous casting of an alloy containing a volatile constituent, some of this constituent distils off the molten and just-solidified metal and is condensed in the pores of the graphite. Whiskers of the volatile constituent then grow in and through the pores and eventually contact and weld onto the hot cast metal. When this occurs, there results the tearing of the die surface whereupon scoring of the surface of the cast metal ensues, producing rough and commercially unacceptable products. Furthermore, the rough die surface offers a greater resistance to the withdrawal of the cast metal from the die and eventually leads to hot tensile breakage of the casting.

Such a phenomenon occurs, for instance, in the continuous casting of nickel silver rod (copper/zinclnickel alloy) wherein some of the zinc constituent, having a high vapour pressure at the casting temperature, distils off the alloy, is condensed in the pores of the graphite, and subsequently welds onto the cast nickel silver rod, leading to tearing of the die surface.

SUMMARY OF THE INVENTION According to one aspect of the present invention, a

die for use in the continuous casting of metal and metalalloys comprises a layer of refractory material providing a die surface, the layer being in intimate contact with and supported by the inner surface of a thermally conductive support, at least the die surface of said layer being non-porous.

The term thermally conductive as used in this specification means that the article concerned is capable of transmitting heat at a rate approximately equal to or greater than those of iron and graphite at the operating temperatures of the die.

Preferably, the layer of refractory material for the die comprises alumina (i.e., Al O although other refractory materials, metallic and non-metallic, may be used which, in addition to producing a non-porous surface,

enable a satisfactory mechanical bond to be achieved between the layer and the support for the support of the layer, and which are inert to the material being cast, for example zirconium triboride, molybdenum, silicon carbide and tungsten carbide.

The die may comprise a layer of refractory material which has been deposited on to a support or, alternatively, a support which has been deposited onto a layer of refractory material.

The support may comprise ferrous or non-ferrous materials, the latter being particularly suitable for dies wherein high thermal conductivity properties are required, provided the materials possess adeuate physical properties such as tensile and compressive strength, and fracture toughness. Ferrous supports enable the refractory layer to be deposited directly thereon at high temperature. Although this may be possible with some non-ferrous supports, in the case of support materials which will not withstand the high temperatures of deposition of the required refractory, a die may comprise a duplex layer of refractory material comprising a preliminary coating of a first refractory material deposited at a first high temperature onto a support, followed by a secondary coating of a second refractory deposited at a higher temperature than said first high temperature onto said first refractory material in order to provide a non-porous die surface.

According to another aspect of the invention a method of manufacturing a die for continuous casting of metals or metal alloys comprises providing a layer for refractory material in intimate contact with and supported by the inner surface of a thermally conductive support to provide a die surface, at least the die surface of said layer being non-porous.

In one form of the invention the method comprises depositing at high temperature a layer of refractory material onto a surface of, and in intimate contact with, a thermally conductive support.

The term high temperature is intended to mean a temperature at which the material being deposited is melted so that impingement of the material with a relatively cool surface of a second material results in the intimate connection of the deposited material with said surface.

In another form of the invention, the method comprises depositing a support of a thermally conductive material around and in intimate contact with an outer peripheral surface of a pre-formed inner refractory layer. The refractory layer may be conveniently initially provided on a disposable mandrel. The mandrel may be removed from the die by machining or by chemical means or by a combination of machining and chemical means. a

A convenient method of depositing the refractory material or the support material, as the case may be, is by plasma arc deposition but other suitable deposition techniques may be employed.

The invention also includes a die when made by the method described above.

A further problem that arises in the continuous casting of metals is the positioning and control of the liquid/solid'interface within the die. For efficient operation of a continuous casting process it is preferable that said interface is located within a length of the die that is surrounded by the cooling medium. I-Iowever, graphite is usually employed for the tube leading from the holding furnace to the die so that, since graphite is a good heat conductor, a considerable amount of heat is abstracted by conduction through the tube wall from the molten metal as it passes along the tube from the holding furnace to the die. Consequently it is difficult to control precisely the location of the liquid/solid interface.

Accordingly, a die for use in continuous casting of metals and metal alloys as defined above is provided with a tube for conducting liquid metal from a holding furnace to the die, the tube comprising a refractory material at least at its inner surface and being a longitudinal extension of the layer of refractory material of the die. Suitable refractory materials for the tube are alumina and zirconia.

The tube may be bonded to an end portion of the refractory layer of the die, or the tube may be formed as a longitudinal extension of the refractory layer of the die. In the latter case, the tube may have an outer shell of refractory material bonded to the outer circumferential surface of said extension.

BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more clearly understood, two embodiments will now be more particularly described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 is an axial cross-section of a die according to a first embodiment, adjacent its associated holding furnace;

FIG. 2 is an axial cross-section of a die according to a second embodiment, adjacent its associated holding furnace.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 of the drawing, in the continuous casting of a nickel silver alloy containing 12% nickel, 26% zinc and 62% copper, a die denoted generally by numeral 1 comprises a die block 2 of copper provided with channels 3 for flow therethrough of cooling water. The die block has a tapered bore 4 which accommodates a tubular support 5 of mild steel of which the periphery is correspondingly tapered to suit the bore 4. The support is formed in two halves which meet along a diametral plane 6. One end of the bore 7 of the tubular support 5 is recessed as at 8 and the whole surface area of the bore, the recess and the end face of the support adjacent the recess is provided with a layer 9 of alumina. This layer is conveniently applied by deposition of the alumina on the semi-circular inner surfaces of each half of the support 5 by a plasma arc torch. In any particular application the thickness of coating required to provide a final non-porous surface may be determined by simple experiment by persons skilled in the art of deposition since it will depend, for instance, upon the velocity of the particles in the spray gun. For a typical bore diameter of -34 inch, an initial deposit of approximately 018L620 inchis reqaiiea'aa'aemiawdf the support 5; the halves are then assembled and the bore is finish ground to leave a layer approximately 01 0/ .0 1 2 inch ihiak'h'avrrig'nan-aoraas'aaasifimh surface.

In order to lead the molten nickel silver alloy from a holding furnace 11 into the die 1, a tube 12 is cast from a mixture of aluminum oxide with addition of 85% orthophosphoric acid (11 1 0 The tube is formed in a steel mould and cured for 24 hours at 400 C followed by firing at 1,700 C to form the crystabalite type structure which is hard and sufficiently stable at the working temperatures involved in the casting of nickel silver.

The discharge end 13 of the tube 12 has a spigot 14 of smaller diameter than that of the coated recess 8 of the support 5. At the inner end of the spigot an integral radially outwardly extending flange 15 is provided to form an abutment surface 16.

The tube 12 is secured in, and projects from, a graphite block 17 mounted in a wall of the furnace 11.

In order to connect the die 1 with the tube 12 the coated recess 8 is located over, and cemented to, the spigot 14 so that the bores of the tube and die are concentric and the end of the support that is provided with a layer of alumina abuts against the surface 16 of the tube. A suitable cement 18 for the purpose is constituted by a fine slurry of alumina and phosphoric acid which produces a reasonable green bond strength by curing at 400 C. Curing is conveniently effected by inserting a small electric resistance heater in the appropriate end of the die. After assembly of the tube 12 and the die 1, and curing of the cement 18, the joint is smoothed and polished so that no excessive resistance to withdrawal of the alloy can occur during casting.

In the use of the die and tube according to this embodiment, upon withdrawing a starter bar from the die, molten nickel silver alloy is discharged from the holding furnace 11 through the tube 12 and into the die 1 where it is solidified and continuously withdrawn in the form of rod.

By virtue of the alumina layer 9 on the surface of the bore of the die, a smooth non-porous die surface is obtained whereby absorption of constituents of the alloy being cast is eliminated. Consequently, smooth castings are produced and they offer little resistance to their withdrawal from the die. Moreover, the refractory layer is thermally stable and withstands the erosive action of the molten alloy. Since the layer is thin compared with the total die section, conduction of heat from the liquid and cast metal is unimpaired.

Arising from the use of non-porous refractory coatings, operating times between die replacements are considerably extended beyond present practice and truly continuous casting is achieved.

Furthermore, the use of a refractory tube to lead the molten alloy from the holding furnace to the dieminimises the amount of heat that is abstracted from the furnace along the wall of the tube to the die and cooling fluid. Consequently greater control may be exercised over the position of the solid/liquid interface of the solidifying alloy within the die. The preferred position of the interface is near the inlet end of the die and this position can be precisely regulated by controlling the flow and temperature of the inlet coolant.

In a second embodiment, illustrated in FIG. 2, an aluminum mandrel 20 having a diameter equal to that of the required bore of a die 21, has deposited thereon a layer 22 of alumina of uniform thickness. The coating is deposited by plasma arc technique and a typical thickness of coating 22 is 0.020 inch on a 34 inch diameter mandrel. The length of the layer is sufficient to accommodate the length of the die 21 and the length of a refractory shell 23. The shell 23 has the same composition and general configuration as the tube 12 of the first embodiment, and comprises a spigot 24 and a flange 25 which presents a circumferential abutment surface 26. The shell 23 is cemented onto the layer 22 at its appropriate position as shown in FIG. 2, so that the shell and its associated length of layer 22 provide a composite tube 27.

A tubular support 28 is built up around the peripheral surface of the remaining length of the pre-formed layer 22 and also over the spigot 24 by spraying copper thereon until a sufficient thickness (typically A inch) has been deposited to allow for subsequent machining of the support to a tapered periphery. The degree of taper is suitable for locating the support in a tapered bore 29 of a die block 30 of copper provided with channels 31 for the flow of coolant therethrough. With this arrangement the support 28 is firmly and integrally connected to the shell 23 simultaneously with the forming of the support. After machining the support the mandrel is removed by axially boring through the mandrel to leave approximately a 0.010 inch wall thickness, and subsequently dissolving out this 0.010 inch thickness of aluminum, for example, in caustic soda, to expose the smooth bore of the alumina layer 22.

The method of construction described in the second embodiment avoids the longitudinal joint along the die formed by the construction of the first embodiment. Moreover, the construction of the second embodiment provides a continuous smooth bore along the whole length of the tube 27 and die 21.

ln modifications of the embodiments, the alumina layer 9,22, has a constant bore diameter but may taper in axial cross-section towards the exit end of the die to improve the cooling characteristics.

The tube 12,27, may be reinforced by incorporating longitudinally extending rods therein of low thermal expansion characteristics, for instance of lnvar (a Registered Trade Mark), a nickel/iron alloy of composition 36% nickel, 0.5% carbon, 0.5% manganese, balance iron.

Further, the tube 27 may be made by winding refractory cloth or string to the required shape.

instead of spraying the complete thickness of the copper support, a thin deposit may first be sprayed onto the alumina layer to provide electrical conductivity, and the remainder of the copper support then built up by electro-deposition. In the case of directly cooled dies it may be advantageous to provide the complete thickness of support by electro-deposition.

I claim:

1. A method of manufacturing a die for the continuous casting of metals and metal alloys, comprising providing a disposable mandrel, depositing a layer of nonporous refractory material upon the peripheral surface of the mandrel for forming a refractory tube, depositing a support of thermally conductive material around and into intimate contact with the outer peripheral surface of said layer about a first circumferential portion thereof to form a tube adapted to contact a cooling device so that the remainder of the refractory material provides a tubular refractory extension, disposing a thick tubular shell of refractory material around and in intimate contact with the exterior of the extension whereby the extension and the shell constitute a tube adapted for securing in a wall of an associated holding furnace, and subsequently removing the mandrel.

2. A method as in claim 1 wherein the thermally conductive material is metal.

' UNITEDISTATES PATENT OFFICE fiERTIFICATE OF CORRECTION Patent No. 3,75 ,7 Dated August 1 1973 Inventor(s) Robert KING e It is certified A that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

IN IT-IE I-HIADING:

After the paragraph? [62] concerningrRelated U.S. Application Data, please insert the following new paragraph:

- FOREIGN APPLICATION PRIORITY DATA [30] November 27, 1967 Great Britain .53849/67."

IN THE SPECIFICATION:

Column2, line 13, "adeuate" should read --edequate-- Column 2, line 31, "for" should read of Signed and sealed this 29th day of January 19% (SEAL) Attest:

EDWARD MELETCHERQJR. '7 RENE D. TEGTMEYER Attesting Officer Acting Commissioner of Patents )RM PO-105O (10-69) USCOMM-DC .0376P09 

2. A method as in claim 1 wherein the thermally conductive material is metal. 